Multiple Ui fixes (#6272)

* PLT-6453 - Improving headings in the web app

* PLT-6414 - Fixing view members modal issue

* PLT-5863 - Fixing alignment in new channel menu

* PLT-6391 - Fixing code block overlay

webapp/components/code_preview.jsx

@@ -118,16 +118,18 @@ export default class CodePreview extends React.Component {
                 <span className='post-code__language'>
                     {`${this.props.fileInfo.name} - ${language}`}
                 </span>
-                <code className='hljs'>
-                    <table>
-                        <tbody>
-                            <tr>
-                                <td className='post-code__lineno'>{strlines}</td>
-                                <td dangerouslySetInnerHTML={{__html: highlighted}}/>
-                            </tr>
-                        </tbody>
-                    </table>
-                </code>
+                <div className='post-code__container'>
+                    <code className='hljs'>
+                        <table>
+                            <tbody>
+                                <tr>
+                                    <td className='post-code__lineno'>{strlines}</td>
+                                    <td dangerouslySetInnerHTML={{__html: highlighted}}/>
+                                </tr>
+                            </tbody>
+                        </table>
+                    </code>
+                </div>
             </div>
         );
     }

webapp/components/new_channel_modal.jsx

@@ -113,7 +113,7 @@ export default class NewChannelModal extends React.Component {
         }
 
         if (this.props.serverError) {
-            serverError = <div className='form-group has-error'><p className='input__help error'>{this.props.serverError}</p></div>;
+            serverError = <div className='form-group has-error'><div className='col-sm-12'><p className='input__help error'>{this.props.serverError}</p></div></div>;
         }
 
         let createPublicChannelLink = (

webapp/components/popover_list_members/popover_list_members.jsx

@@ -177,8 +177,7 @@ export default class PopoverListMembers extends React.Component {
                         className='more-modal__row'
                         key={'popover-member-more'}
                     >
-                        <div className='col-sm-3'/>
-                        <div className='more-modal__details'>
+                        <div className='more-modal__details text-center'>
                             <div
                                 className='more-modal__name'
                             >